1. Field of the Invention
The present invention relates to the field of information communication systems. More particularly, the present invention relates to the field of secure communication modes for information representing audio/visual works.
2. Related Art
In recent years, there has been a technique, by which a plurality of audio/visual (AV) devices can be connected using digital interfaces so that AV information (e.g., representing AV works such as movies, songs, etc.) can be transmitted from an information source (e.g., a video disk player or video cassette recorder) to an information display device (e.g., a television set or monitor) or an information sink device. The development of this technology has led to the adoption of the IEEE 1394 serial communication standard for AV devices. Within the IEEE 1394 serial communication standard, information is transmitted in digital packets having certain header information and having data sections.
The AV digital information that is transmitted (e.g., representing movies) is usually protected by a copyright to prevent unauthorized users from viewing the information and making unauthorized reproductions of the AV works. To prevent unauthorized copying, the AV information is transmitted with encoded copy control information called the Copy Control Information (CCI) bits. The CCI bits can also be referred to as Copy Generation Management System (CGMS) bits. The encoded CCI code is made up of 2 bits, of which "00," "10," and "11" indicate "unlimited allowance for copies," "allowance for a single generation copy," and "inhibition of copy," respectively. The "01" bit code is left unused. The manner in which the CCI bits are added to an AV information is specified for the different kinds of AV information (e.g., Motion Picture Expert Group MPEG, Digital Video DV and audio data). These CCI bits are added within the data portion of data packets that constitute the AV information to control the use of the AV information by down stream devices.
At each data recording time, a recording device inspects the CCI code added to the AV information of a packet and refrains from recording them if the CCI code indicates the copy prohibition. If the CCI code indicates the allowance of a single copy, the CCI code is changed to copy prohibition mode and then the AV information is recorded in a recording media once. Thus, the copy generations allowed from the original data are limited.
Moreover, in order to enforce the copy generation limiting system, there has been employed a method by which CCI codes are encrypted into the data portions of the information and the decryption information is licensed to the manufacturer that has made a contract to manufacture only a device conforming to the copy generation limiting system. In order for the recording device to inspect or alter the CCI code encrypted within the AV information, it is necessary to provide a microcomputer mounted on the device or to provide specialized hardware for the processing. In order to match numerous kinds of AV information, the required decryption circuitry is relatively complex, increasing the cost of the device.
For an inexpensive recording device, e.g., a bit stream recording (BSR) device, it is conceivable to manufacture the device without the ability to read the CCI codes within the AV information because the above specialized hardware is eliminated. For this inexpensive BSR device, it has been conceived to provide a unique field within the AV packets to store copy protection information. FIG. 1 illustrates fields of a prior art information packet 10 including a header section 12 in accordance with the IEEE 1394 standard, a packet (CIP) header section 14 and a data field portion 16. The data packet is an isochronous packet. A special bit stream copy control information (BCI) field 20 is included within the CIP header section 14. The BCI field 20 contains the copy protection information for use when AV information is transmitted to a BSR device. Although the data section 16 can contain encrypted data, the CIP header section 14 is typically transmitted without encryption to accommodate the BSR's limited capabilities.
In response to this packet 10, the BSR device inspects the BCI field 20 of the packet header 14 and does not record the data if the packet indicates the copy inhibition, but can record the data if the same indicates the single generation copy allowance or the unlimited allowance. If allowed, the packet information can be recorded by the BSR device together with the BCI code 20 which is stored in the packet header 14. Provided the original packet header indicated the single generation copy allowance, when the recorded information is reproduced and supplied from the BSR device to the IEEE 1394 bus the recovered BCI code 20 will indicate copy inhibition (because it has already copied once). However, the same BCI code is stored and transmitted if the original BCI code indicated no copy limit.
FIG. 2 illustrates a system 30 in which the above copy protection can be compromised by an unauthorized device 34. As shown, a source device 32 transmits information to a sink device 36, which is a BSR device. An intercept device 34 is placed between this communication path (38a and 38b). The BCI code 20 in the packet header 14 can be corrupted by intercept device 34 while the packet is being transmitted from the transmitting device 32 to the receiving device 36. For instance, the transmitting device 32 transmits the BCI code 20 having the bit code "11" indicating the copy prohibition, but it is corrupted (by device 34) during the transmission into the bit code "10" indicating the single generation copy allowance or can be altered to the bit code "00" indicating no copy restriction at all. In response to this packet, the BSR 36 is not informed of the fact that the data intrinsically should inhibit the copy, and records the packet information because the BCI code of the packet header allows a single generation copy or unlimited copies. Therefore, the copy generations cannot be controlled.
Accordingly, what is needed is a copy protection system whereby packet information can be transmitted from a source device to an unintelligent device without compromise by an intermediate device. What is further needed is a copy protection system whereby packet information can be transmitted from a source device to a BSR device without compromise by an intermediate device. What is needed further is a system that does not allow copy protection mode information within a transmission to be altered and yield usable results at the sink device. The present invention provides such advantageous features. These and other advantages of the present invention not specifically mentioned above will become clear within discussions of the present invention presented herein.